Inorganic sheet laminate

ABSTRACT

A laminate of inorganic sheet and polyester resin having an overall thickness of 5 to 25 mils and having an elongation at break in both the cross direction and machine direction of at least 25%.

BACKGROUND OF THE INVENTION

The present invention is directed to a improved laminate of inorganicsheet and polyester polymer, preferably a laminate of two inorganicsheets separated by a polyester polymer layer. Such laminates are usefulin transformers and other electrical devices wherein the laminate servesas dielectric insulation material. Such laminates need to have foldingendurance and toughness. Any improvement in the internal adhesion of thelaminate or the tear or elongation at break properties of such laminatesis desirable.

SUMMARY OF THE INVENTION

The present invention is directed to a laminate of inorganic sheet andpolyester resin having an overall thickness of 5 to 25 mils and havingan elongation at break in both the cross direction and the machinedirection of at least 25%. Preferably, the elongation at break in thecross direction is at least 30%. In one embodiment of this invention,the thickness of the resin layer in the laminate can be greater than thethickness of any individual nonwoven sheet in the laminate. It ispreferred the nonwoven inorganic sheet be a paper and that the paperinclude an inorganic mineral such as aluminum silicate, an inorganicreinforcement such as glass fiber, and a binder such as acrylonitrilelatex. The preferred polyester resins useful in the laminate arepoly(ethylene terephthalate) and poly(ethylene naphthalate).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified representation of an extrusion lamination processuseful in making the laminates of this invention.

FIG. 2 is a representation of the improvement in initial tear resistanceof the extrusion laminates of this invention over prior art adhesionlaminates.

DETAILED DESCRIPTION OF THE INVENTION

Laminates made from inorganic sheets or papers and polyester resin filmshave been used in transformers and other electrical devices wherein thelaminate serves as dielectric insulation material. It is desired thatsuch insulative laminates have a combination of physical propertieswhich are especially suited for the needs of transformer manufacturers.These properties include in addition to insulative properties, othermechanical properties which include initial tear resistance (as measuredby elongation at break) and high resistance to tear propagation (asmeasured by average tear load). These properties are especially usefulin evaluating insulative laminates because in the manufacture oftransformers there is the likelihood the insulative laminate will bedamaged during assembly.

It has been found that the elongation and tear properties of inorganicinsulative laminates can be improved by replacing the form of thepolyester that is used in the laminates. In particular, it has beenfound that a laminate made with a molten polyester resin has improvedelongation and tear properties over laminates made with films.

Typically, laminates used in the prior art for electrical insulationhave utilized polyester film. Since polyester film by itself does nothave good adhesion to inorganic sheet, adhesives have been used toattach the films to the inorganic sheet. The films were attached to theinorganic sheet by first coating an adhesive onto the film and thenlaminating the coated film onto the inorganic sheet at high temperature.It is believed that the use of polyester in film form in the laminatelimits the elongation and tear properties of the final laminate, in thattypical processes for forming a solid film impart crystallinity andorientation into the polyester layer. This is believed to reduce theflexibility, toughness, and tear resistance of the final laminate.

The laminates of this invention utilize inorganic sheet or paper, theterms meant to be used interchangeably herein. The sheets or papersutilized in this invention can be prepared using conventionalpaper-making processes and equipment and processes. The terms “crossdirection” and “machine direction” are well-known in the art and referto orthogonal directions in the sheet where physical properties aremeasured; the machine direction runs parallel with the windup directionof the paper machine and the cross direction traverses the papermachine. The inorganic papers of this invention can contain a mineralcomponent, an inorganic reinforcing component, and a binder. Thesecomponents are combined in a water slurry and cast onto a screen wherethey are dewatered, optionally pressed, and dried. Layers of thesesheets can be combined to build a certain thickness of paper, and suchpapers can be further consolidated by calendaring or other densifyingprocesses. The mineral and/or inorganic material useful in the making ofthe inorganic papers and sheets used in this invention include but arenot limited to glass, aluminum silicate, and/or mixtures of these withother inorganic fillers; such inorganic materials may be present infiber form. Useful binders include, but are not limited to,acrylonitrile latex. The preferred laminates of this invention are madefrom CeQuin® and TufQuin® inorganic sheets available from Quin-TCorporation, Tilton, N.H.

The thickness of the inorganic sheet is not critical and is dependentupon the end use of the laminate as well as the number of inorganiclayers employed in the final laminate. Although the present inventionmay employ two layers, i.e. one inorganic layer and one polymer layer,and preferably employs three layers, i.e. two inorganic sheet layers andone polymer layer, it is understood that there is no upper limit in thenumber of layers or other materials which can be present in the finalarticle.

As employed herein the term inorganic means that the primaryconstituents are non-hydrocarbon clays, fibers, flakes, platelets orother structures, particularly naturally-occurring minerals such asaluminum silicate, and processed inorganic materials such as glassfiber.

The preferred polymers applied to the inorganic sheet in this inventionare polyethylene terephthalate (PET) and polyethylene naphthalate (PEN).These polymers may include a variety of comonomers, including diethyleneglycol, cyclohexanedimethanol, poly(ethylene glycol), glutaric acid,azelaic acid, sebacic acid, isophthalic acid, and the like. In additionto these comonomers, branching agents like trimesic acid, pyromelliticacid, trimethylolpropane and trimethyloloethane, and pentaerythritol maybe used. The PET may be obtained by known polymerization techniques fromeither terephthalic acid or its lower alkyl esters (e.g. dimethylterephthalate) and ethylene glycol or blends or mixtures of these. PENmay be obtained by known polymerization techniques from 2,6-naphthalenedicarboxylic acid and ethylene glycol.

One method of making the laminates of this invention is by extrudingmolten polymer between two inorganic sheets followed by pressing andquenching to form the laminate. The molten resin can be extruded ontothe inorganic sheets in any number of ways. For example, the resin maybe extruded onto one inorganic sheet and then covered with a secondinorganic sheet and then laminated using a press or laminating rolls.Referring to FIG. 1, in a preferred method, the molten resin is suppliedto a slotted die 1 from an extruder. The slotted die is oriented so thata sheet of molten resin is extruded in a downward fashion to a set ofhorizontal laminating rolls 2. Two supply rolls of inorganic sheet 3provide two separate webs 4 of inorganic sheet to the laminating rollsand both webs and the sheet of molten resin all meet in the nip of thelaminating rolls with the resin positioned between the two webs. Therolls consolidate the webs and resin together; the consolidated laminateis then quenched using a set of cooled rolls 5. The laminate may then becut to appropriate size as needed for the application.

In another embodiment of this invention, the combination of moltenpolymers may extruded in a manner which layers the different polymersbetween the two inorganic sheets. For example, the polymer layer couldconsist of three layers such as, in order, a layer of PET polymer havinga first intrinsic viscosity, a layer of PET polymer having a secondintrinsic viscosity, and a third layer of PET polymer having the sameintrinsic viscosity as the first layer. In this manner a PET polymerhaving more affinity to inorganic sheets can be employed to incorporatea PET polymer having less affinity to inorganic sheets into thelaminate.

The laminates of this invention have a thickness of from 5 to 25 milsand have an elongation at break in both the cross and machine directionsof at least 25%. The preferred laminates of this invention have across-direction elongation at break of at least 30% and anmachine-direction elongation at break of at least 25%. Generally suchlaminates will have a resin thickness greater than any one inorganicsheet in the laminate.

In the following examples all parts and percentages are by weight unlessotherwise indicated. Initial tear resistance was measured via elongationat break per ASTM D1004. Tear propagation resistance was measured viaaverage tear load by ASTM D1938.

EXAMPLE 1

This example illustrates some of the problems encountered in thelamination of inorganic paper to polyester polymer without adhesives andthe adhesion attained by this invention. Two types of sheets are used inthis example; an inorganic paper in an as-purchased condition(specifically TufQuin® manufactured by Quin-T Corporation), and acarrier sheet of meta-aramid paper which was highly calendered so thatmolten polyester polymer would not adhere to it.

A heated press was used to laminate poly (ethylene terephthalate)polyester having an intrinsic viscosity of 0.60 dl/g betweencombinations of the two sheets of papers as below. No adhesive,chemical, flame, heat, or corona treatment or similar activation of thepaper surface was required for the polyester polymer to adhere to theinorganic paper. Heated press conditions are shown in Table 1.

Test A—Small pellets of polyester polymer were placed between twoinorganic sheets and the combination placed in the heated press. Thepolyester pellets melted and flowed between the sheets. The binder usedin inorganic sheet also melted and flowed, dissolving the combination ofsheets and polymer into an unusable laminate.

Test B—Small pellets of polyester polymer were placed between twoinorganic sheets and the combination placed in the heated press, butheld for a shorter period of time than for Test A. The polyester pelletsmelted and flowed between the sheets. The binder used in inorganic sheetalso melted and flowed but the combination of sheets and polymerretained its integrity. However, the laminate was brittle andeffectively unusable.

Test C—Small pellets of polyester polymer were placed on one meta-aramidsheet in a heated unclosed press. After five minutes in the heatedunclosed press, the polymer pellets had melted and flowed on surface ofthe aramid sheet. After the aramid sheet and molten polyester polymerwere removed from the press, and while the polymer was still molten, onesheet of inorganic paper was manually pressed onto the surface of themolten polyester polymer. The laminate was immediately quenched toprevent melting of the binder used in the inorganic sheet. Good adhesionbetween inorganic sheet and polyester polymer was noted withoutembrittlement of the inorganic sheet.

TABLE 1 Test: 1 2 3 Paper Type two sheets two sheets one sheet each ofof inorganic of inorganic aramid paper and paper paper inorganic paperPress Temperature (° C.) 288 288 288 Time in Press (min) 5 2.5 5 Timeout of Press (min) 0 0 5 Polymer Thickness (in) 0.005 0.005 0.005

EXAMPLE 2

This example illustrates the properties of the laminates of thisinvention made by extrusion lamination, versus laminates made byadhesive lamination. The extrusion laminates were made as follows.Inorganic paper (specifically CeQuin® manufactured by Quin-TCorporation) was used as purchased. Molten poly(ethylene terephthalate)(PET) polymer in the form of a three-layered combination or co-extrusionof PET polymers having intrinsic viscosities of 0.65/0.85/0.65 dl/g,respectively, was applied to the surfaces of the paper by extrusionlamination of polyester polymer between the two papers. No adhesive,chemical, flame, heat, or corona treatment or similar activation of thepaper surface was required for the polyester polymer to adhere to theinorganic paper. These extrusion laminates were compared to commerciallyavailable adhesive laminates used in electrical insulation containing apolyester film adhesively laminated between two inorganic papers(CeQuin® laminates manufactured by and commercially available fromQuin-T).

Samples of laminate produced in this manner were tested per ASTM D1004(“Test Method for Initial Tear Resistance in Plastic Film and Sheeting”)and ASTM D1938 (“Test Method for Tear Propagation Resistance of PlasticFilm and Sheeting by a Single-Tear Method”) to assess the tearproperties achieved.

In all tests, the adhesive bonding between the polyester polymer and theinorganic paper was greater than the internal strength of the paper. Theresulting data in Table 2 illustrates that laminates of this inventionmade by extrusion lamination had both improved initial tear resistance,as measured by having an elongation to break of greater than 30% in thecross-direction and greater than 25% in the machine direction, alongwith improved tear propagation resistance. As used below, EL stands forextrusion lamination, AL stands for adhesive lamination, MD stands formachine direction, and XD stands for the cross or traverse direction.FIG. 2 illustrates the improvement in elongation to break for theselaminates, with the lines 10 and 15 representing the MD and XD valuesfor the extrusion laminates and lines 20 and 25 representing the MD andXD values for adhesive laminates.

TABLE 2 Type of Laminate AL AL AL EL EL Inorganic Sheet Thickness (mils)3 3 3 3 3 Polymer Thickness (mils) 4.5 7.4 9.4 8.4 10.4 MD Elongation atBreak (%) 15 19 19 32 36 XD Elongation at Break (%) 14 18 18 33 35 MDAverage Tear Load (lb-f) 1.0 1.9 2.2 3.6 4.0 XD Average Tear Load (lb-f)1.3 2.0 2.2 4.1 4.6

1. A laminate of inorganic sheet and polyester resin having an overallthickness of 5 to 25 mils and having an elongation at break in both thecross direction and machine direction of at least 25%.
 2. The laminateof claim 1 wherein the elongation at break in the cross direction is atleast 30%.
 3. The laminate of claim 2 wherein the thickness of the resinin the laminate is greater than the thickness of any individualinorganic sheet in the laminate.
 4. The laminate of claim 1 wherein theinorganic sheet is a paper comprising an inorganic mineral, an inorganicreinforcement, and a binder.
 5. The laminate of claim 4 wherein theinorganic mineral includes aluminum silicate.
 6. The laminate of claim 4wherein the inorganic reinforcement includes glass fiber.
 7. Thelaminate of claim 4 wherein the binder includes acrylonitrile latex. 8.The laminate of claim 1 wherein the polyester resin is poly(ethyleneterephthalate).
 9. The laminate of claim 8 wherein the poly(ethyleneterephthalate) includes a comonomer selected from the group ofdiethylene glycol, cyclohexanedimethanol, poly(ethylene glycol),glutaric acid, azelaic acid, sebacic acid, and isophthalic acid.
 10. Thelaminate of claim 8 wherein the poly(ethylene terephthalate includes abranching agent selected from the group of trimesic acid, pyromelliticacid, trimethylolpropane, trimethyloloethane, and pentaerythritol. 11.The laminate of claim 1 wherein the polyester resin is poly(ethylenenaphthalate).
 12. The laminate of claim 1 wherein the polyester resin issandwiched between two inorganic sheets.
 13. The laminate of claim 12wherein the polyester resin sandwiched between two inorganic sheetsincludes a layered combination of resins.
 14. An electrical devicecontaining an electrical conductor and an insulating laminate comprisinginorganic sheet and polyester resin, said laminate having an overallthickness of 5 to 25 mils and having an elongation at break in both thecross direction and the machine direction of at least 25%.
 15. Theelectrical device of claim 14 wherein the laminate has an elongation atbreak in the cross direction of at least 30%.
 16. The electrical deviceof claim 14 which is a transformer.